This fixes the limitation that prevents fio from doing random I/O with zones
enabled.

This also adds a 'zonerange' configuration option which may be too ambiguous
and is subject to potential renaming in the future.

When doing random I/O, it is beneficial to be able to specify how large the
addressible space is in the zone, while specifying a different metric for
how much data to read from that zone (i.e., how many samples to take from each
zone). When 'zonerange' is not specified, it defaults to be equal to the
'zonesize' option. When both are specified, 'zonerange' indicates the
size of the zone while 'zonesize' indicates the quantity of data to read from
each zone.